This invention relates to a method of refining NF.sub.3 gas containing CF.sub.4 as impurity by using an adsorbent.
With an extensive development in the manufacture of semiconductors and related devices there is a growing demand for a wider selection of etching gases for the manufacture of semiconductors and cleaning gases for cleaning the manufacturing apparatus or the products.
Recently NF.sub.3 gas is attracting attention as an etching gas for use in plasma etching because of the capability of greatly enhancing the etching rate and also because of leaving no solid residue on the surface of the etched material. In the case of using CF.sub.4 gas as an etching gas there is a problem about a solid residue. Besides, NF.sub.3 gas is useful as a cleaning gas for cleaning semiconductors and some parts of apparatus for manufacturing semiconductors by, for example, CVD process.
As shown in U.S. Pat. No. 4,543,242, NF.sub.3 gas can be prepared safely and economically by the reaction of fluorine gas with ammonium cryolite, (NH.sub.4).sub.3 AlF.sub.6. It is expected that NF.sub.3 gas prepared by this method will be used for wide purposes in increasing quantities.
For use in the field of semiconductors, NF.sub.3 gas of very high purity is required. When NF.sub.3 gas is prepared by the aforementioned method it is inevitable that the obtained gas contains a very small amount of CF.sub.4 which originates in fluorine gas used as a reactant. So, there is the need of separating CF.sub.4 from the obtained NF.sub.3 gas, but the separation is not easy. Both NF.sub.3 and CF.sub.4 are very low in chemical activity at normal temperature, and the boiling point of CF.sub.4, -128.degree. C., is very close to the boiling point of NF.sub.3, -129.degree. C. Therefore, at present the separation is carried out by a distillation method utilizing a difference between NF.sub.3 and CF.sub.4 in vapor pressures, but this method is very low in efficiency.